Dual Transfer Mode (DTM) is specified in 3GPP TS 43.055 V6.7.0 (2004-11), 3rd Generation Partnership Project; Technical Specification Group GSM/EDGE, Radio Access Network; Dual Transfer Mode (DTM); Stage 2 (Release 6). In this dual transfer mode, where the mobile station (MS) has simultaneous Circuit-Switched (CS) and Packet-Switched (PS) connections, a handover procedure is defined only for the CS resources. The details of a handover of CS resources is being specified for general handover purposes in 3GPP TS 23.009 V6.0.0 (2004-12), Technical Specification, 3rd Generation Partnership Project; Technical Specification Group Core Network; Handover procedures (Release 6).
In DTM handover, the PS resources are torn down and only re-established once the MS has successfully completed the handover. The handover of PS resources is being standardized for general purposes in the 3GPP TSG (Technical Specification Group) “GERAN” (GSM (Global System for Mobile Communication) EDGE (Enhanced Data Rates for GSM Evolution) Radio Access Network) in the technical specification 3GPP TS 43.129 v6.0.0 (2004-11), 3rd Generation Partnership Project; Group GERAN, Packet-switched handover for GERAN A/Gb mode; Stage 2 (Release 6).
FIG. 1 of TS 43.129 shows a reference architecture for PS handover in GERAN A/Gb mode including a Core Network (CN) with an MSC connected to a first SGSN via a Gs interface, the first SGSN connected to a second SGSN and to a GGSN via Gn interfaces, the MSC also connected to a PSTN with the first SGSN connected to other networks via a Gp interface, and with the GGSN connected to other networks via Go and Gi interfaces. For the radio access network, a BSS/GERAN is shown connected to the MSC of the CN via an A interface and to the first SGSN via a Gb interface. The BSS/GERAN includes a BSC connected to BTSs by Abis interfaces. One of the BTSs is shown in communication with an MS via a Um (wireless) interface. An RNS (or another BSS) is also shown with an RNC (or BSC) connected to NodeBs each having cells with Uu interfaces also capable of communicating with the MS. The RNC (or BSS) is shown connected to the MSC via an Iu-CS interface and to the second SGSN via an Iu-PS interface. The figure shows both user traffic and signalling on most of these interfaces. It will be understood from the foregoing that in the case of a handover involving both CS and PS resources, there is a longer path for the PS signalling which will take longer than the CS signalling.
As suggested above, the DTM procedures specified in 3GPP TS 43.055 only enable the handover of CS resources, while PS resources have to be (re-)established after the MS's successful access in the new cell.
It has been shown that for such an MS in DTM, the PS service interruption at handover is substantial (3.4 s in the Inter-BSC Inter-MSC Inter-SGSN case, with approximately 2.7 s induced by Routing Area Update procedure). See PS Service Interruption Estimations for DTM Operation, 3GPP TSG GERAN #17bis, Edinburgh, Scotland, 12-16 Jan. 2004, Source: Siemens; Tdoc G2-040049, Agenda Item 5.4.6.2. This interruption is not tolerable for services with stringent delay requirements, e.g., PS conversational video. Thus, the handover in the existing TS 43.055 specification introduces a problematic interruption of the PS service.
While Dual Transfer Mode (DTM) Enhancements have been proposed as a work item for GERAN A/Gb mode in 3GPP Release 6 (see Reduction of PS service interruption in Dual Transfer Mode in TSG GERAN #17, Budapest, Hungary, 17-21 Nov. 2003 in Tdoc GP-032548, Agenda item 6.7, see also GERAN Project Scheduling and open issues for GERAN in 3GPP GERAN TSG#22, Cape Town, 8-12 Nov. 2004 (GP-042836)), there is still an outstanding need to define improvements enabling combined handover of the CS and PS resources in dual transfer mode in order to minimize the end-to-end delays for real-time PS services. The service interruption required by the real-time PS services is a maximum of 150 ms.
There is no prior art solution enabling combined (parallel or simultaneous) handover of CS and PS resources.
The problem could be circumvented if the handover of the PS resources were to be enabled together with the handover of the CS resources either in parallel or simultaneously.
A problem in enabling the combined handover of CS and PS resources in parallel in DTM is that the CS and PS handover procedures are executed independently of each other and hence there might be a risk that one procedure completes before the other, for example, CS handover is completed prior to the PS Handover completion. This would lead to failing a handover for one of the CS or PS resources in DTM, although there were resources allocated in the target cell, the DTM handover would not be able to be performed. Therefore, there is a need to synchronize the PS and CS handover procedures in the source and target systems, such that the MS in DTM can continue with both the CS and PS sessions after handover at the same time.